Actually, he is. A read/option guy doesn't have to complete 73% of his passes. He just needs to be able to hit open receivers consistently. Tebow seems to be a little lacking there but he could fill in for a half or a game without overhauling the entire offense because RGIII, Kap, Newton, etc, etc got dinged.
Seriously, look at the back-ups in the NFL. Do you want Derek Anderson behind the wheel? Mike Glennon? Tavaris Jackson? Dominique Davis? GTFO.
But, FWIW, I do understand why teams don't want Tebow on their bench. He has a following. As I've said, repeatedly, your #1 has to be a no-doubter and your coach has to be a "STFU, I decide who's QB" kind of guy. SF actually looks like a good fit but they have Colt McCoy.